Many demodulators used in digital communication devices use zero-crossings to determine the polarity of the demodulated signal. In zero IF (Intermediate Frequency) applications, signal phase information can be extracted from the zero crossings of the in-phase and quadrature signals. These zero-crossings are detected by an appropriate circuitry to demodulate the carrier signal and reconstruct the originally transmitted information signal.
In general, digital signals may be demodulated in various ways. In U.S. Pat. No. 4,322,851 issued on Mar. 30, 1982, Ian Vance teaches a method of demodulating a binary FSK signal. This technique uses the direction of the phase rotation angle to detect signal polarity. A significant problem with this approach is its inability to demodulate multi-level digital signals.
To overcome this limitation, a Cross-Differentiate-Multiply (CDM) demodulation technique may be employed. This technique is detailed in an article titled "An FM Detector for low S/N" written by John Park and published in IEEE T-Comm, Vol. com-18, No. 2, April 1970. This technique is highly complicated and requires two differentiators, four multipliers, one divider and two summers. The number of devices, hence their current drain, used to realize this technique is prohibitive particularly in battery operated portable communication applications.
Yet another approach employs the tan.sup.-1 (inverse tangent) operation {[q(t)]/[i(t)]} with digitized i and q signals. This technique requires two Analog to digital converters and a processor. Once again the current drain is prohibitive, particularly in portable applications.
A demodulation technique is therefore desired that could efficiently demodulate multi-level digital signals.